lecture 17 regulation of the cell cycle and cell death
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Lecture 17Regulation of the Cell Cycle
and Cell Death
The Cell Division Cycle
A cell contains all the information necessary for making a copy of itself during cell division
Cells reproduce by carrying out a highly ordered sequence of events called the CELL CYCLE
– Typically Involves:Cell GrowthDNA ReplicationDistribution of Duplicated Chromosomes to Daughter CellsCell Division
Phases of the Eukaryotic Cell Cycle
Different Cell Types have Variations in Timing, Control and Progression Through Cell Cycle
In the Adult Human Body
-Most Cells Are Not Actively Dividing
-Some Cells Are Continuously Dividing- Example: Bone Marrow Cells, Epithelial Cells
Some Cells Do Not Divide After Differentiation
Example: Nerve Cells
Some Cells can Be Triggered to Divide to Replace Cells that have Died
Example: Cells of Liver Tissue
Control of Cell Cycle Involves Monitoring
Intracellular And Extracellular Conditions
The Control System Needs to Regulate Progression through the Cell Cycle to Ensure:
1)Events Associated with Each Phase are Carried Out at the Appropriate Time and in the Appropriate Order
2) Each phase is complete Before Next Phase is Initiated
3) Must be Able to Respond to External Conditions Required for Cell Growth and Division.
A Series of Control Points called Checkpoints Accomplish These Objectives
Cell Cycle Checkpoints Regulate Progression Through the Cell Cycle
Cell Fusion Experiments Provide Evidence for Control Molecules in the Cell Cycle
.
S
S S M M
MG1 G1
S Phase is ActivatedIn Original G1 Nucleus
M phase is ActivatedIn Original Non-M Nucleus
The Cell Cycle Is Controlled by Cyclin Dependent Kinases (CDK)
CDK forms a Complex with a Protein called a Cyclin to form a CDK Complex
Different Cyclin-CDK Complexes Trigger Different Steps in the Cell Cycle
4 Major Types of Cyclin-CDK Complexes: G1-CDKG1/S-CDKS-CDK M-CDK
Regulation of the Activity of CDK Complexes
1) Proteolysis of Cyclin Component
2) Phosphorylation/Dephosphorylation
3) Binding of Inhibitory Proteins
4) Subcellular Localization
Cyclin Protein Concentration and Cyclin-CDK Activity Oscillate during the Cell Cycle
Post Transcritional Control of Cyclins:M-Cyclin is Ubiquitinated and Subsequently
Degraded by the Proteasome
Cyclin Destruction Occurs at Specific Points during the Cell Cycle
Checkpoints in the Cell Cycle
Progression through the G1 Checkpoint
Cells Need to Check for : Cell SizeNutrientsMitogens and Growth Factors DNA Damage
Called the Restriction Point in MammalsCommits Cell to the Process of Cell Division
G1 checkpoint
G1G1
G0
Cells Can Delay Cell Division by Entering Specialized Nondividing State, GO
Green Light to Proceed: Environment Favorable
Red Light – Don’t ProceedEnvironment Unfavorable
(Absence of Mitogenic Signals)
Most cells in our body are In Go State:
In the Absence of Mitogenic Signals,
The protein Rb Inhibits Cell Cycle Progression Rb- Retinoblastoma Protein
Binds Transcription Factor E2F and Prevents Function
E2F Required for:Activating TranscriptionOf Genes Encoding ProteinsRequired for G1/S transition
G1/S Cyclins, S Cyclins andComponents of DNA
Replication Machinery
The G1-CDK Complex Controls the G1 Checkpoint by Phosphorylating the Rb Protein
G1 DNA Damage Checkpoint
DNA Damage Needs to be Repaired before DNA Synthesis Occurs
Activation of Transcription Factor p53 leads to synthesis of CDK inhibitor (CKI), p21
p21 binds and inhibits G1/S CDK and S-CDK Complexes
Cells Arrest Until Damage is Repaired and Then Progress
Or
Undergo Apoptosis if Unable to Repair
p53 Mediates the G1 DNA Damage Checkpoint Through Inactivation of G1/S- and S-CDK Complexes
Checkpoints in the Cell Cycle
S-CDK Complexes Are Required for DNA Replication During S-phase
S-CDK Controls:1) Initiation of Replication 2) Prevents Re-replication from a Particular Origin
Progression Through the G2 Checkpoint to Enter Mitosis
Check for:Cell SizeDNA Replication Complete
Passage through G2 Checkpoint Requires Active M-CDK:
Functions to Phosphorylate Proteins Involved in Early Stages of Mitosis
1) Nuclear Envelope Breakdown2) Chromosome Condensation3) Mitotic Spindle Formation 4) Targeted Protein Degradation
Unreplicated DNA SensedBlocks Activating
Phosphatase
Unreplicated DNA Blocks Activation of M-CDK Complex
T
Checkpoints in the Cell Cycle
M-CDK Controls The Spindle Assembly Checkpoint by
Activating the Anaphase Promoting Complex (APC)
Check for:
Proper Chromosome Attachment to Spindle
Phosphorylation of APC- now Activated
Sister Chromatids Can Separate
If Chromosomes not Properly Attached
Metaphase Arrest
Will not Separate
Exit From Mitosis
Now Need to Reverse Events:Nuclear Envelope BreakdownChromosome CondensationMitotic Spindle Formation
Destroy M-CyclinM-CDK Activates APC
APC Targets M-Cyclin for Destruction by the Proteasome
Cancer Involves Defective Cell Cycle Control Mechanisms
Cancer Inducing Mutations Inherited or Introduced by Viruses
Two Key Types of Mutated Genes That Can Lead to Cancer:
Oncogenes - Gene whose presence can trigger development of cancer.
Example: ras, bcl-2 Tumor Suppressors- Gene whose absence or
inactivation can lead to cancerExample: p53, Rb
Mitogens and Growth Factors Activate the Ras Pathway Leads to Passage through the G1 Checkpoint
p53 Can Induce Apoptosis in
DNA Damaged Cells
Can’t RepairDNA DAMAGE
Apoptosis
Apoptosis:Regulated Cell Death
Role in Killing of Unneeded, Damaged, or Potentially Deleterious Cells
Occurs in Embryonic and Adult Tissues
Proteins Involved are Always Present in Cells- Needs to Be Activated by Stimuli
Can Result From:
Developmental Cues
Withdrawl of Essential Growth Factors
DNA Damage
Various Cell Stresses
Programmed Cell Death
• Cell Death Occurring at a Defined Point in Development
• Usually proceeds by Apoptosis
Mouse Paws
Not All Cell Death is Apoptotic
Apoptosis:
An Active Regulated Process
DNA FragmentationChromatin Condensation
Fragmentation of NucleusCell Shrinks
Formation of Membrane Enclosed Fragments called Apoptotic Bodies
Recognition and Engulfment by Phagocytic Cells
or Neighboring Cells
Oncosis and Necrosis:
Unregulated Cell Death Due to Injury
Cell Swells (Oncosis)
Nucleus Swells
Disruption of Organelles and Rupture/Release of Contents
Contents Released into Extracellular Space
Necrosis and Apoptosis are Morphologically Distinct
Necrosis Apoptosis
The Morphological Changes of Apoptosis Are Orchestrated by Caspases
Cysteine Proteases that cleave at Aspartic Acid Residues
Activate Apoptosis by Cleaving Specific SubstratesPresent but inactive in cells
Two Main Types of Caspases1) Initiators- Need to dimerize to become active “induced proximity”
2) Executioners- Need to be proteolytically cleaved to become active- Cleavage is usually Mediated by Initiator Caspases
Once Executioners are Activated their Key Targets of Proteolysis Include:
1)An Inhibitor of a DNAse- Fragmentation of DNA
2)Nuclear Lamins- Fragmentation of Nucleus
3)Other Cytoskeletal Associated Proteins- Disruption of Cytoskeleton and Cell Fragmentation
4)Activation of Additional Caspases
Caspases are Present but Inactive in Cells
Zymogens: Proteins initially synthesized as inactive precursors- undergo proteolytic cleavage to become active
Caspase Activation Amplification Cascade
So How Are Initiator Caspases Activated to get the Process Going??
Main Pathways Regulating Caspase Activation During Apoptosis
Intrinsic Pathway- Mitochondrial MediatedMajor Pathway in Mammalian Cells – Outer Mitochondrial Membrane Permeabilization (MOMP)– Release of Cytochrome C from Mitochondrial Intermembrane
Space into Cytosol– Apoptosome Formation- Activation of Initiator Caspase – Effector Caspases Activated
Extrinsic Pathway- Signaling through Death Receptors– Ligand Bound Death Receptors– Adaptor Protein Association– Initiator Caspase Recruitment and Activation– Effector Caspases Activated
Intrinsic Pathway of Apoptosis Activation
Association of Adaptor with Procaspase allows
Procaspase self cleavage
MOMPs
cytochrome c Release
Apoptosome Formation: Adaptor (Apaf1), dATP
cytochrome c and procaspase complex
Active Initiator CaspaseCleaves Effector
Caspases Which now Cleave
Targets
Critical Regulators of Cell Death
Bcl-2 Family – Regulate whether MOMPs Occurs
Anti-Apoptotic Factors - Death Inhibitors
A) Function to Inhibit Cytochrome C release
Pro-Apoptotic Factors- Death Activators A) Bind and inhibit Death Inhibitors
B) Directly cause Permeabilization of Stimulate Release of Cytochrome C ( BAX AND BAK)
IAP Family (Inhibitor of Apoptosis)Bind Procaspases prevent activation
Bind Caspases and inhibit Activity
Survival Factor Signaling is Required to Prevent Apoptosis
Programmed Cell Death in Neuronal
Development
Survival Factors Signaling Keeps
Death Inhibitor Bcl-2 Active
No Survival SignalBcl-2 Complexes with BadCan’t preventBAK and BAX MediatedMOMPs
Extrinsic Pathway of Apoptosis Activation:Signaling through the Death Receptors
Ligand Bound Death Receptors
Adaptor Protein and Procaspase Recruitment
Initiator Caspase Activation
Effector Caspases Activated
Target cells :
Viral Infected Cells or Cancer Cells
Removal of Excess Lymphocytes afterInfection